This invention relates to devices for contacting a flowing gas or vapor stream with a flowing liquid stream, the liquid and vapor being contained in a surrounding vertical containment vessel. Thru use of such devices, the liquid streams can remove gaseous or liquid constituents from the gas streams and transfer same to the liquid streams. The purpose is to recover valuable chemical products from the gas stream or to purge the gas stream of pollutants, such as solid particulates, sulfuric or hydrochloric acid vapors or mists, sulfur compounds such as sulfur dioxide or trioxide, or nitrogen compounds such as nitrous or nitric oxide, the gas stream often consisting of combustion products generated by burning gas, oil or solid fossil fuels, containing elements giving rise to such pollutants in the flue gas stream. Alternatively, the gas liquid contactor can be used for cooling a hot liquid stream with a cold gas stream, as in the case of a water cooling tower. In this device, a stream of hot water is contacted with an air stream flowing countercurrent to the water stream. The air enters the tower at low temperature and humidity, the humidity being the water vapor concentration in the air in pounds of water per pound of dry air, and leaves at higher temperature and humidity. The water evaporating from the liquid stream increases the humidity of the air, and causes the hot liquid to cool by an amount equivalent to the latent heat of vaporization of the evaporated water. The cooled water can be used in one or more heat exchangers, wherein the cooled water is used to cool a hot chemical process stream, and in so doing is reheated and returned to the cooling tower. The water is continuously circulated in a closed loop, the loop consisting of the heat exchangers, the cooling tower, a circulation pump and an interconnecting piping system.
Conventional spray type gasxe2x80x94liquid contactors, often referred to as absorption towers or scrubbers, operate by dispersing the liquid stream in the form of small droplets, that provide a large amount of surface for mass transfer, and ensure intimate contact between the gas and liquid. Dispersion of the liquid stream can be achieved by passing pressurized liquid thru a conventional spray nozzle, having a small exit opening, that imparts high velocity to the exit stream. The nozzle design is such, as to direct the high velocity liquid exit stream against a secondary stream surrounding the exit stream, so that impaction and liquid dispersion of the exit stream can occur. Dispersion of the exit stream can also be made to occur, by impacting the exit stream against a stationary target appended to the nozzle. Contactors using such nozzles suffer from a disadvantage in that the spray exiting the nozzles has a conical shape, having an included angle of 15 to 90 degrees. As a result, the contact time between gas and liquid varies from 0 at the widest point of the spray to a maximum at the centerline of the spray. If the widest point of the spray exceeds the vessel diameter, the sprayed droplets will impinge upon the walls of the enclosing vessel, coalesce, and the available droplet surface will be drastically reduced, as will the contacting efficiency between gas and liquid. Furthermore, the orifice diameter of the spray nozzle is relatively small, usually less than 1 inch, so that if the liquid or gas feed is contaminated with fouling substances, the nozzle orifices can plug and cause inoperability of the contactor. The spray contactor does, however, have the advantage of operating at very low gas pressure drop, as compared with other types of contactors. Another such contactor, in common use, is the packed tower. This type of device contacts gas and liquid in a vessel containing packing, specifically designed for the purpose, or consisting of crushed stone, ceramics or the like. In operation, the liquid breaks up and flows around the surface of the packing, so as to provide a large surface area for gas-liquid contacting. Unfortunately, because of the small clearances between individual packing particles, the packing is prone to plugging, high pressure drop, and inoperability, should the liquid or gas be laden with fouling substances. Even in the absence of plugging, the packed tower tends to operate at higher gas pressure drop than does a spray tower of equal size, thereby increasing the operating cost of devices needed to move gases thru the tower.
The gasxe2x80x94liquid contactor, which is the object of this invention, overcomes the afore mentioned problems with contactors in present use, by eliminating the need for packing, in the case of packed towers, and by generating a liquid spray of small liquid droplets, having high surface area per unit of tower volume, in the case of spray type contactors. High liquid pressurization is not used, in conjunction with the subject invention, feed liquid instead being dispersed with air or gas at low pressure and low velocity, usually less than 1 inch of water and 100 feet per second respectively. This is accomplished using nozzles of relatively large diameter, usually much greater than 1 inch, which are highly resistant to plugging by fouling substances in the contactor feed.
The present invention relates to a gasxe2x80x94liquid contactor, the embodiment of which is shown in FIG. 1. The design is such as to intimately contact a vertical downward flow of small diameter droplets, with a vertical upward flow of gas. Such a flow arrangement is termed countercurrent, and is the flow arrangement preferred, because the average driving force for mass transfer of matter from the gas to the liquid, or from the liquid to the gas, is greatest for this flow arrangement. Although countercurrent flow is normally preferred, co-current flow, wherein both the gas and liquid flow upward or downward is not to be excluded as a possible arrangement, to be used in conjunction with the subject invention.
Downward flow of liquid droplets is achieved by means of a multiplicity of low pressure, gas assisted, liquid spray nozzles, located at an appropriate elevation between the main inlet and outlet feed gas nozzles of the contactor containment vessel. Each spray nozzle consists of a vertical tube or conduit, 1 or more inches in diameter and xc2xd inch or more in length. Liquid feed is introduced at the base of each nozzle, and flows upward and around the outside of the nozzles until reaching the open ends at the top of the nozzles. The liquid then overflows the tops of the nozzles, which are all at the same elevation, and flows downward at the inner surfaces of the nozzles, in the form of more or less continuous films and/or rivulets. The liquid flow rate, in the case of low viscosity fluids such as water, is typically less than 1000 pounds per hour per nozzle. Air or a portion of the feed gas is introduced at the top of each nozzle, in a total amount for the nozzle grouping equal to less than 50% of the total feed gas flow, and flows downward in the same general direction as the liquid feed. In so doing gravitational forces, in addition to shear or drag forces, exerted by the flowing gas on the liquid streams, results in the formation of large liquid droplets and filaments at the base of the nozzles. When these forces exceed the surface tension of the liquid, the filaments are dispersed as small droplets. These droplets are further reduced in size, typically to xe2x85x9 inch or less, by virtue of shear forces generated by relative movement between the droplets and the surrounding counter-flowing feed gas stream. In contrast to the conical droplet profile assumed by conventional pressurized liquid spray nozzles, the flow path of liquid droplets assumed by the low pressure gas assisted spray nozzles, are very nearly vertical and parallel, so that the average gasxe2x80x94liquid contact time, between all elements of the upward flowing gas stream, and downward flowing droplets is the same, and equal to the maximum contact time. This is to be compared with an average contact time, in the case of pressurized liquid spray nozzle usage, equal to 50% of maximum or less. As a result, the height of a contactor utilizing gas assisted liquid spray nozzles, will be correspondingly smaller than one using pressurized liquid spray nozzles.
Gas outlet nozzles, equal to or greater in diameter than the gas assisted spray nozzles and equal to or greater in number than the spray nozzles, surround the spray nozzles in a pattern such that the droplets discharged from the base of each spray nozzle is contacted by an equal quantity of the up-flowing feed gas. Velocity of the feed gas entering and leaving the gas outlet nozzles is typically less than 100 feet per second. The base of each gas outlet nozzle and each spray nozzle are at the same vessel elevation, with the top free end of each spray nozzle terminating in a separate compartment receiving the liquid feed. The top free end of the gas outlet nozzles terminate at a higher elevation, and in a region separated from the spray nozzle liquid feed reception compartment.
Large diameter nozzles route the total feed gas flow, the total liquid feed flow, and the total spray nozzle assist gas flow into and out of the contactor containment vessel. Nozzles are sized on the basis of pressure drop limitations, typical velocities being 50 feet per second or less for gases and 10 feet per second or less for liquids. More than a single nozzle may be used for each service should size restraints so demand.